Over the past several decades automotive vehicle manufacturers have satisfied continually-decreasing limits on the amounts of carbon monoxide, unburned hydrocarbons, and nitrogen oxides (collectively, NOx) that are discharged to the atmosphere in the exhaust from vehicle engines. These requirements of reduced exhaust emissions are combined with requirements for increased fuel economy. These combined requirements have required ever more sophisticated engines, computer control of engines, and exhaust gas treatment systems, including catalytic reactors, in the exhaust stream.
Present exhaust treatment systems are quite effective in treating the exhaust from a warmed-up engine because the catalyst materials have been heated to temperatures (e.g., 250° C. and above) at which they serve to effectively oxidize carbon monoxide and incompletely burned fuel constituents to carbon dioxide and water, and to reduce nitrogen oxides to nitrogen. These treatment systems have been quite effective for both gasoline-fueled engines operating at a stoichiometric air-to-fuel ratio and diesel engines (and other lean-burn engines) which operate with considerable excess air (sometimes called “lean burn”). It has been difficult to treat exhaust emissions immediately following a cold engine start, before the exhaust has heated the catalytic reactor or reactors to the effective temperatures of the catalyst materials. It is realized that such untreated emissions will make-up a significant portion of the total emissions in the mandated testing of engine exhaust systems. The problem is particularly difficult with the treatment of mixed nitrogen oxides in the exhaust of diesel engines. These nitrogen oxides comprise nitric oxide (NO) and nitrogen dioxide (NO2), the mixture typically referred to as NOx. There is, therefore a need for better systems for treating the exhaust gas from an engine following a cold start. The need is particularly acute in lean-burn engines, such as diesel engines, which tend to produce cooler exhaust streams because of the excess air used in the combustion mixtures charged to their cylinders.